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docs(script): Expand on dataset selection mechanism (#1)

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hygienic-books 2023-05-06 19:52:16 +02:00
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@ -38,7 +38,12 @@ Hook files from both directories are collectively parsed and executed in lexicog
For ZFS snapshots intended to save your bacon the `00-*` naming convention is particularly critical. In `/usr/share/libalpm/hooks` you can see for example that when a kernel upgrade happens `60-mkinitcpio-remove.hook` is executed (deleting your existing `vmlinuz-*` kernel image for example at `/boot/vmlinuz-linux`). After that if you're using the `zfs-dkms` package which itself requires `dkms` which in turn installs `71-dkms-remove.hook` this hook removes your ZFS kernel module files. Both the `60-*` and optionally the `71-*` hook (for `zfs-dkms` users) run early due to their naming. If we don't create a snapshot before them we end up creating a snapshot that has not kernel image and no ZFS kernel module files. Out `00-*` hook files are executed first ensuring that a snapshot can safely return you to a working system. For ZFS snapshots intended to save your bacon the `00-*` naming convention is particularly critical. In `/usr/share/libalpm/hooks` you can see for example that when a kernel upgrade happens `60-mkinitcpio-remove.hook` is executed (deleting your existing `vmlinuz-*` kernel image for example at `/boot/vmlinuz-linux`). After that if you're using the `zfs-dkms` package which itself requires `dkms` which in turn installs `71-dkms-remove.hook` this hook removes your ZFS kernel module files. Both the `60-*` and optionally the `71-*` hook (for `zfs-dkms` users) run early due to their naming. If we don't create a snapshot before them we end up creating a snapshot that has not kernel image and no ZFS kernel module files. Out `00-*` hook files are executed first ensuring that a snapshot can safely return you to a working system.
The _trivial_ snapshot chain by default keeps 15 snapshots, the _important_ chain keeps 5. The thought process here is that you will likely not futz around with a kernel every day whereas you may very well install arbitrary packages multiple times a day. Snapshots should keep you safe for a couple of days hence the defaults of 5 and 15 snapshots, respectively. By default we identify the active system dataset by doing `findmnt / --noheadings --output source` which for example returns:
```
zpool/root/archlinux
```
If exactly one source returns that is the exact name of a ZFS dataset in an imported zpool we create a snapshot on it. If no source returns we silently exit. If more than one source returns we raise an error and halt the `pacman` transaction.
Snapshots may look like so: Snapshots may look like so:
``` ```